The present invention relates to helicopter rotor blades of the type having a hollow pressurized spar whose pressure is monitored for detection of leaks caused by flaws in the spar. More specifically, this invention relates to rotor blades of this type in which the hollow spar is covered by an impervious layer, or skin, provided with inner ducting to collect and discharge any gas which would leak from a flaw in the spar.
By the prior art, the skin for such helicopter rotor blades having hollow pressurized spars has been made of impervious fiberglass material which is wrapped, as a "blanket", around the spar. To allow the gases passing through a flaw in the spar to escape, slot-like ducts have been provided in the inner side of this so-called blanket, so that the internal pressure of the spar may fall, to indicate the flaw, such as shown in U.S. Pat. No. 3,765,124, issued to Donald L. Ferris in 1973.
By one conventional process the fiberglass lay-up method is used to fabricate spar blankets. A sheet of fiberglass material is positioned on a table and a plurality of spaced-apart layers of strips of fiberglass are stacked on the fiberglass sheet, leaving slots, which become the venting ducts, between the strips. The fiberglass lay-up is then placed within a vacuum bag and cured in an autoclave. Since each strip of fiberglass must be carefully cut and set into place, this method is quite time-consuming. And, while flat lands (the surfaces between grooves or slots) are obtainable adequate dimension tolerances are difficult to achieve.
As an alternative to the inadequate layup method the use of a grid to form the grooves was attempted. However methods of protecting lands from mold release agents resulted in a spar blanket having rounded corners on the lands which are to contact the rotor blade spar. The rounding of these land corners so reduces the bonding area that a wholly satisfactory bond is not obtained. Rotor blades are subjected to numerous types of loadings, e.g. torsion, flap, thrust, etc. Consequently all of the blade components must be strongly bonded into an integral unit. With this in mind slotted fiberglass blankets have been fabricated by machining slots in a previously-cured blanket. However this process is too time consuming, and, it is almost impossible to keep surfaces to be bonded sufficiently clean. Soiled surfaces weaken the bond, and a weak bond impairs the integrity of the blade.
An object of this invention is to provide a better method of fabricating a fiberglass blanket having a plurality of grooves on one of its sides for use in overlaying and venting the outer surface of a pressure sealed hollow helicopter rotor spar. The process results in a reduction of fabrication time, improves the dimensional tolerances of the blankets, and still satisfies bonding requirements.